No. 1 swimming pool problem? It's number two!

A lifeguard keeps watch on opening day of the McCarren Park Pool, June, 2012 in Brooklyn. CDC researchers found evidence of feces in many US pools.

People always worry about pee in the pool, but number two is the No. 1 problem, government health experts say. They found plenty of evidence that someone’s pooping in the pool.

It’s not only disgusting, but it’s evidence that people are not following basic hygiene rules, says Michele Hlavsa, chief of the Centers for Disease Control and Prevention’s Healthy Swimming Program.

“It is time to stop treating the swimming pool as a toilet,” Hlavsa told NBC News. “Nowhere else except for the pool is it acceptable to poop in public or pee in public. In other places if we did this in public, we’d be arrested.”

CDC and county health officials tested the pools in four Georgia counties last summer for the study. They got water out of the pool filters and ran tests for microbes – especially E. coli, which lives inside the bodies of animals and which is carried in feces. If E. coli is somewhere, it likely got there via feces.

“Escherichia coli, a fecal indicator, was detected in 93 (58 percent) samples; detection signifies that swim­mers introduced fecal material into pool water,” Hlavsa’s team wrote in CDC’s weekly report on disease and death.

“We don’t know how it got in there. It either washed off somebody’s body or somebody had a bowel movement in the pool,” Hlavsa said.

Regular bowel movements – what CDC delicately refers to as a “formed fecal incident” – isn’t so much a worry. But diarrhea is. Diarrhea carries the germs that caused the illness right into the water, and the chlorine doesn’t kill it instantly.

The good news is that there were no reports of sickness outbreaks at any of the pools tested last summer – or anywhere else in Georgia last summer, for that matter, Hlavsa says. But every year there are between 20 and 80 disease outbreaks associated with public pools – and those are just the ones that get reported, Hlavsa says.

The other piece of good news is that there weren’t too many other pathogens in the pools. Only one sample of Cryptosporidium, a common cause of diarrhea, showed up. Two samples of a parasite called Giardia were found – that one can cause a very uncomfortable stomach upset. Viruses such as adenovirus, which causes colds, and norovirus, which causes "stomach flu," can survive in swimming pools.

The E. coli in the pool didn’t necessarily come from a bowel movement, Hlavsa said -- and this is why it’s important to actually obey the signs that virtually everyone ignores about showering before going into the pool.

“The average person has about .14 grams of feces on their rear end,” Hlavsa said. “If that rinses off into the water, the amount from one person might not be that much. But as more and more swimmers introduce it that much, it does become an issue.”

She’s actually done the math.

“Let’s imagine 1,000 kids go to a water park. They have as much as 10 grams of feces on their rear ends,” she said. “We are now talking about 10,000 grams or 10 kg. That translates to 24 pounds of poop in the water.”

So one important rule – never swallow the water from a swimming pool. “Basically, these pools are big bathtubs we all share together,” Hlavsa said.

She doesn’t want to scare anyone away from swimming, which is, after all, a great way to exercise. “As soon as the weather gets warm enough here in Atlanta, I plan to swim,” Hlavsa said.